In recent years, out of consideration for the terrestrial environment, magnesium alloys (hereinafter referred to as “magnesium materials”) are employed for reinforcements making up an engine, a frame, etc. of a vehicle, for purposes of enhancing fuel efficiency of the vehicle, for example.
The magnesium materials have been attracting attention as materials having a number of practically excellent properties in application for structural materials. To be more specific, magnesium is a metal material that is practically lightest in weight (e.g., specific gravity thereof is approximately two thirds of aluminum and approximately one fourth of iron), stronger and stiffer than iron and aluminum, having highest capability in absorbing vibrations (damping capacity) among practical metal materials, highly resistant to dint, less likely to undergo change in dimension with time or according to variation of temperature, and easily recyclable. For that reason, the magnesium material is suitable, in particular, for a structural material for vehicles, and for a housing of portable terminals.
However, in cases where a magnesium material is employed for a structural material of a vehicle or the like to be used in a high-temperature atmosphere, especially, when the magnesium material is employed for members making up an engine, which could be exposed to a high temperature approximating 200° C., an axial (tightening) force of a bolt, for example, would disadvantageously decrease in a portion where members are fastened with the bolt.
Decrease in axial force of the bolt in such a bolt-fastened portion may take place due to deformation of a fastened surface of the member or a nut, and it has been conceived that the decrease in axial force of the bolt would particularly depend upon the creep strength of the material.
As a result, in order to prevent the decrease in axial force of the bolt, various kinds of magnesium materials (magnesium alloys) having improved creep strength have been developed.
For example, a refractory magnesium alloy containing aluminum, zinc, or the like each in a specific proportion and formed by adding silica, rare-earth metal, calcium and the like is known in the art.
However, with a conventional magnesium alloy as above, though some improvement in creep strength can be achieved, it is difficult to check the decrease in axial force of the bolt on the condition that a member made up of the magnesium alloy is used in a relatively high-temperature atmosphere, and thus the aforementioned disadvantages cannot be deemed to be finally rooted up.